The hydrogenation of coal is generally carried out by pasting it with a heavy oil and heating the resultant paste in a hydrogen containing atmosphere. High temperatures and high hydrogen partial pressure must be used to achieve hydrogenation; the magnitude of the temperature and pressure required depending on the degree of hydrogenation required. The temperature and pressure requirements can, however, be reduced by the use of catalysts and both homogeneous and heterogeneous catalysts are known.
Heterogeneous catalysts may be supported or unsupported, and each type has its advantages and disadvantages. Supported heterogeneous catalysts, e.g. Co--Mo supported on an aluminum base, tend to be poisoned by the impurities, e.g. V and TiO.sub.2, present in the carbonaceous material, they are abraded by the solid carbonaceous feed material, they can be blinded by coke formed during hydrogenation and they also require the use of mechanically sophisticated, and therefore expensive, vessels to carry out the reaction. Unsupported heterogeneous catalysts, e.g. MoO.sub.3, Fe.sub.2 O.sub.3 and FeS, are either poor catalysts or they are expensive and therefore have to be recovered to achieve economic operation. Catalyst recovery from the ash and unreacted feed materials has in the past proved to be extremely difficult. Homogeneous catalysts, i.e. catalysts that dissolve in pasting oil, include transition metal carbonyls and hydrocarbonyls (e.g. Co.sub.2 (CO).sub.8), Zeigler catalysts (e.g. nickel naphthenate reduced by triethyl aluminum), and Lewis acids e.g. AlCl.sub.3, SnCl.sub.2, and SbF.sub.5.
The Lewis acid catalysts are disadvantageous because they are corrosive. Generally the other homogeneous catalysts are to a greater or lesser degree unstable at hydrogenation temperatures and pressures and they, like unsupported heterogeneous catalysts, must often be recovered to achieve economical operation. On the other hand, unsupported heterogeneous and homogeneous catalysts can be used in simple furnaces, e.g. plug flow reactors, and they are not fouled by impurities or coking.
In order to liquefy solid carbonaceous material it is generally necessary to use temperatures in the 400.degree.-500.degree. C. range since at lower temperatures, liquefaction does not take place and at higher temperatures, excessive coking occurs. Even within the preferred temperature range some coking often occurs. Coking should be avoided because it reduces the yield of liquid hydrocarbon, which is the most valuable liquefaction product. Coke formation should also be avoided when the catalyst is used in more than one hydrogenation, such a cyclical operation will result in a coke build-up. A reduction of coke formation during hydrogenation makes the operation of the equipment much easier. Furthermore, at these high temperatures, many catalysts, particularly the homogeneous transition metal carbonyl catalysts, decompose. Also many metal carbonyls vapourise at liquefaction temperatures and pressures, thereby reducing their effectiveness as a catalyst.
It would be desirable to provide an effective catalyst that is cheap and that is available in such quantities that there would be no need to recover it from the hydrogenation product and so could be discarded. Cheap catalysts are known e.g. pyrrhotite, which is a non-stoichiometric mineral form of iron sulphide, Fe.sub.2 O.sub.3, FeSO.sub.4 and Luxmasse, which is a red mud containing Fe.sub.2 O.sub.3, TiO.sub.2 and Al.sub.2 O.sub.3, but such catalysts are not very effective in promoting liquefaction and preventing coking reactions.
U.S. Pat. No. 4,325,802 ('802) teaches an aqueous method of liquefaction of carbonaceous materials using a metal carbonyl and water gas under alkaline (above 7.5 pH) conditions to form a mixture. The mixture is heated to obtain the hydrocarbon liquids.